--- plan_stats.orig.h	2021-05-03 21:32:31.309222261 -0700
+++ plan_stats.h	2021-05-03 20:16:17.854204705 -0700
@@ -33,6 +33,7 @@
 #include <cstdlib>
 #include <string>
 #include <vector>
+#include <optional>
 
 #include "mongo/db/index/multikey_paths.h"
 #include "mongo/db/jsobj.h"